Rubber foams have excellent cushioning properties and therefore are useful for such applications as cushioning materials and padding materials. As well known, in comparison between closed cells and continuous cells in foam structures, the former has a structure having cells separated in a steric lattice form by partitions and the latter has a structure in which partitions located between a series of cells separated by the partitions of the closed cell structure are removed; the latter can be deformed dynamically more easily. However, although the effect of water blocking and air sealing is out of the question for continuous cells, closed cells are expected to have the effect of water blocking and air sealing due to the presence of partitions between the cells, but a foam with closed cells hardly conforms to gaps because it hardly deforms dynamically due to its high facial pressure. As a solution, it is known to use a water-swellable polymer foam as a sealing material for tunneling work or work of water and sewage construction and a water-blocking material for engineering and architecture work, many proposals regarding the composition and the production method of such water-swellable polymer foams have been made (see JP-A-6-65410, JP-A-6-25380, JP-A-7-109369, JP-B-5-48775 and JP-B-7-68396).
A technology involving passing an item through a gap between a pair of rolls rotating in the same direction to apply a compression force and a shearing force simultaneously has also been proposed (see WO 2006/043570 A1).
Sealing materials made of the water-swellable polymer foams disclosed in JP '410, JP '380, JP '369, JP '775 and JP '396 are insufficient in initial sealing properties to be exerted after coming into contact with water and until the sealing effect due to water-swellability is exhibited completely. If a foam mainly containing closed cells is used to satisfy the initial sealing properties, not only the sealing effect due to water-swellability fails to be exhibited satisfactorily, but also it becomes difficult to install the foam into a complicated gap due to its low deformability.
The above-described foam disclosed in WO '570 is economically disadvantageous because of the use of a method in which a crosslinked polyolefin resin foam sheet is produced by a publicly known and used process and then the sheet is processed by subsequent processing, and the foam is insufficient also in terms of the sealing properties at the time of being used as a sealing material.
In view of such background of the existing technologies, it could be helpful to provide a crosslinked polyolefin resin foam which is flexible and exhibits excellent sealing properties.